Emission regulations for diesel vehicles have been dramatically strengthened to reduce air pollutions of urban area. Korea has introduced European strict regulations to light-duty diesel vehicles which Euro6 emission limit was enforced from 2014. Recent studies of on-road NOx emissions have shown that considerable NOx discrepancies exist in driving certification test cycle and on-road. It is regarded as a reason of ineffectiveness of strict vehicle emission regulations on reducing air pollutants such as NO2. In this study 20 Euro6 vehicles have been tested in various driving conditions on chassis dynamometer beside conventional certification driving cycle, NEDC. The results have shown very large range of NOx emission values as tested vehicles and engine calibration rather than emission reduction technologies impacted on increased NOx emissions. When vehicles were driven in high ambient temperature with operating air conditioner and in low ambient temperature, reduced EGR function caused excessive NOx emissions. A vehicle emitted high NOx emission in slightly different driving conditions caused by shut-off EGR and was considered to use defeat device.